1. Field of the Invention
This invention relates to a dry-process toner for developing an electrostatic image in image forming processes such as electrophotography, electrostatic recording and electrostatic printing.
2. Related Background Art
It is conventionally known to form an image on the surface of a photoconductive material by an electrostatic means, and develop it by the use of a toner.
A large number of methods are known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, an electrostatic image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently a toner is caused to adhere to the electrostatic image to form a toner image.
Next, the toner image is transferred to the surface of an image holding medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. A copy or print is thus obtained. In the case when the process comprises a toner-image transfer step, the process is usually provided with a cleaning step for removing the toner remaining on the photosensitive member.
As developing methods by which the electrostatic latent image is developed by the use of a toner, known methods can be exemplified by the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these developing methods, colorant-containing resin particles (toner particles) are commonly used which are prepared by melt-kneading a thermoplastic resin and a colorant, thereafter cooling the kneaded product obtained and finely pulverizing the cooled kneaded product. The thermoplastic resin most commonly includes polystyrene resins. Besides, polyester resins, epoxy resins and acrylic resins are also used. As a non-magnetic colorant, carbon black is widely used. In the case of magnetic toners, black magnetic powders of an iron oxide type are used. In a system in which a two-component type developer is used, the toner is usually used admixture with carrier particles such as iron powder and ferrite powder or resin-coated carriers of these.
The toner image finally formed on an image holding medium such as paper is fixed onto the image holding medium by the action of heat, pressure, or heat-and-pressure. In this fixing step, the step of fixing by a heating pressure means has been conventionally widely used.
In recent years, rapid progress is being made from monochromatic copying or monochromatic printing to full-color copying or full-color printing, and research has been conducted on two-color copying machines or full-color copying machines, which have been widely put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p.52 (1986) have published reports relating to color reproducibility and gradation reproducibility.
In color image formation carried out by full-color electrophotography, three color toners of the three primary colors, yellow, magenta and cyan are commonly used optionally together with a black toner to reproduce all colors.
The full-color copying process comprises the steps of forming an electrostatic image on a photoconductive layer by passing light reflected from an original, through a color-separating light-transmitting filter having the relation of complementary color to the color of a toner, followed by developing and transfer, through which a color toner image is held on an image holding medium. These steps are repeated several times to superimpose toner images of respective colors on the same medium while making registration, followed by fixing carried out once to obtain a final full-color image.
In general, in the case of a development system making use of a two-component developer comprised of a toner and a carrier, the toner is electrostatically charged to the desired charge quantity and charge polarity by its friction with the carrier, and the electrostatic attraction force produced is utilized to develop electrostatic images by the use of a toner having triboelectric charges. Accordingly, in order to obtain good visible images, the triboelectric chargeability of toner must be maintained at a good level.
Presently, to meet such problems, charge control agents and fluidity-providing agents employed in toners and binders serving as a toner matrix are selected to achieve superior triboelectric chargeability for developers containing them.
For example, to add a charging auxiliary such as chargeable fine particles to the toner, Japanese Patent Publication No. 52-32256 and Japanese Patent Application Laid-Open No. 56-64352 disclose a technique of adding a resin powder having a polarity reverse to the toner, and Japanese Patent Application Laid-Open No. 61-160760 discloses a technique of adding a fluorine-containing compound to developers so as to achieve a stable triboelectric chargeability.
To add such a charging auxiliary, for example, it is common to use a method in which electrostatic attraction force or van der Waals force, acting between toner particles and the charging auxiliary, is utilized to cause the latter to adhere to the toner particle surfaces by using a stirrer or a mixer. In such a method, however, it is not easy to uniformly disperse the charging auxiliary on the toner particle surfaces. Charging auxiliary particles not adhering to toner particles may form agglomerates, tending to result in an increase in the quantity of agglomerates brought into a free state, as liberating from toner particles. This tends to more significantly occur with an increase in specific electrical resistance of the charging auxiliary and with a decrease in particle diameter. Where the free agglomerates are present in a large quantity, they may influence toner performance therefrom. For example, the toner, may have an unstable amount of triboelectricity at the time of extented copying, tending to result in non-uniform image densities and formation of images with much fog.
In addition, the content of the charging auxiliary changes when copies are continuously taken, to make it difficult to keep image quality at the initial stage.
There is a method in which a charge control agent is previously added together with a binder resin and a colorant when toners are produced. However, the amount of the charge control agent added or the amount of its presence on the toner particle surfaces can not be controlled with ease because it is not easy to uniformly disperse the charge control agent in the binder resin and also because what substantially contributes to charging performance are those charge control agents present in the vicinity of toner particle surfaces and those present inside the particles do not contribute to the charging performance.
It is proposed to externally add fine titanium oxide particles to toner particles to impart fluidity to the toner and stabilize charging.
Since, however, as disclosed in Japanese Patent Application Laid-Open No. 60-112052, an anatase type titanium oxide having a low volume resistivity is used, triboelectric charges may leak quickly especially in an environment of high humidity, improvements have had to be made especially in respect of stabilization of charging. In addition, the anatase type titanium oxide has, in the X-ray diffraction described later, an intensity ratio Ia/Ib larger than 12.0 which is a ratio of maximum intensity Ia to minimum intensity Ib within the range of 2.theta.=20.0 to 40.0 deg.
Japanese Patent Application Laid-Open No. 5-72797 (corresponding to EP-A-523654) discloses a proposal relating to a toner containing a hydrophobic amorphous titanium oxide. Since, however, the amorphous titanium oxide has lower abrasive properties than crystalline titanium oxide, an improvement has had to be made with respect to the member abrasion of photosensitive member surface and removal of deposits on the photosensitive member surface. Since the amorphous titanium oxide also has many OH groups even after hydrophobic treatment, an improvement has had to be made in respect of charging performance which may become reduced because of adsorption of water content especially in an environment of high humidity. The amorphous titanium oxide has an intensity ratio Ia/Ib smaller than 5.0.
Japanese Patent Application Laid-Open No. 6-332232 also discloses a proposal to add an acicular or needle-like titanium oxide with a large major axis particle diameter. However, the toner has a low fluidity which is greatly affected by the acicular shape and large major axis particle diameter of the titanium oxide. This acicular titanium oxide has an intensity ratio Ia/Ib exceeding 12.0.
Japanese Patent Application Laid-Open No. 6-332233 also discloses a proposal relating to a toner to the particle surfaces of which titanium oxide particles represented by TiO.sub.x (x is less than 2) are deposited. These titanium oxide particles, however, are black or blue and are unsuitable as an external additive for a color toner such as yellow toner or magenta toner. Also, because of their relatively large particle diameter, they have a low performance in imparting fluidity to toner and also tend to scratch the photosensitive drum surface. The TiO.sub.x where x is less than 2 commonly has an intensity ratio Ia/Ib larger than 12.0.
Japanese Patent Application Laid-Open No. 5-188633 also discloses a proposal relating to a toner containing a fine powder of hydrophobic-treated anatase type titanium oxide. Since this titanium oxide has perfect anatase crystals, titanium oxide particles may mutually agglomerate in part to scratch the photosensitive drum surface, or, when externally added to small-diameter toner particles, the toner may have a low fluidity. This anatase type titanium oxide has an intensity ratio Ia/Ib larger than 12.0.
Some titanium oxides are known to have rutile type crystals, which, however, have a small BET specific surface area. Their crystals have grown in an acicular or columnar shape, and hence may impart fluidity and abrasive properties at an undesirably low level.
It is eagerly awaited to develop titanium oxide particles providing more improvements in sufficient fluidity, charging performance, abrasive properties, environmental stability and running performance for toners.
Moreover, in recent years, there is an increasing commercial demand for making copying machines or printers achieve a higher resolutior and making images have a higher quality. In the present technical field, it has been attempted to make toner particle diameter smaller so that a color image can be formed in a higher image quality. Making smaller the particle diameters of toner particles results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This tends to cause an insufficiency of image density or a deterioration of running performance of the toner. In addition, because of the large quantity of triboelectricity, toner particles may strongly adhere to one another to cause a decrease in fluidity, bringing about a problem in the stability of toner feed and the imparting feed triboelectricity to the toner feed.
In the case of non-magnetic color toners, they contain no conductive materials such as magnetic materials and carbon black, and hence the toner particles have no portions from which triboelectric charges are leaked, and typically tend to have a larger quantity of triboelectricity of the toner. This tends to occur especially when polyester type binders having a high charging performance are used.
Color toners are desired to satisfy performances as shown below.
(1) Fixed toners are required to nearly form into a substantially completely molten state to the extent that the forms of toner particles can not be recognized, so as not to obstruct their color reproduction because of irregular reflection upon exposure to light. PA0 (2) Color toners must have a transparency high enough to perform subtractive color mixing well with an underlying toner layer having a color tone different from its upper toner layer. PA0 (3) The respective color toners must have well-balanced hues and spectral reflection properties, and sufficient chroma.
From such viewpoints, resins of a polyester type are widely used as binder resins for color toners used to form full-color images. Toners comprised of polyester resin as binder resin, however, commonly tend to be affected by temperature and/or humidity, and tend to cause problems of an excessive charge quantity in an environment of low humidity and an insufficient charge quantity in an environment of high humidity. Thus, it is eagerly awaited to provide color toners having stabler charging performance even in a wide range of environment.